Group choices when expanded is true

Hello,

Actually choices are grouped when using the `group_by` capability only if expanded is `false`.
It should be also possible to group choices also when expanded is `false`:

``` php
<p>Group 1</p>
// list widget_choices for group 1
<p>Group 2</p>
// list widget_choices for group 1
```

http://symfony.com/doc/current/reference/forms/types/entity.html#group-by
